This invention relates to an .alpha.-cyanoacrylate adhesive composition improved in fluidity and physical properties of the cured product, particularly a composition which was made thixotropic or gelled by use of a silica gel treated with an alkylsilyl isocyanate.
An .alpha.-cyanoacrylate represented by the formula: ##STR1## (wherein R.sup.1 is alkyl, aryl, alkenyl, aralkyl, haloalkyl, alkoxyalkyl having 1 to 18 carbon atoms, etc.) polymerizes within a short time with moisture or an anionic material to bond various materials firmly and is therefore used as an instantaneous adhesive, as is well known in the art.
This .alpha.-cyanoacrylate adhesive is characterized in that it is readily flowable with low viscosity, having the property of being easily used for penetration adhesion or automatic coating.
However, on the other hand, it readily splashes out from a vessel, thereby sometimes causing accidents during handling. This has been an obstacle in its practical application, and there has been a great need for its improvement.
As methods for meeting this need, a method in which viscosity is increased by use of a thickener and a method in which the adhesive is made thixotropic or gelled by use of various plastic fluidizing agents have been proposed.
For example, Japanese Laid-Open Patent Publication No. 43247/1979 discloses an adhesive thickened by use of a colloidal inorganic compound, particularly ultra-fine silica in .alpha.-cyanoacrylate, and also Japanese Laid-Open Patent Publication No. 107993/1979 proposes an adhesive composition produced by use of an inorganic or organic plastic fluidizing agent including silicon dioxide in .alpha.-cyanoacrylate. Otherwise, in U.S. Pat. Nos. 4,477,607 and 4,533,422, an .alpha.-cyanoacrylate adhesive composition comprising use of a silica gel, particularly a fumed silica modified with polydimethylsiloxane or trialkoxyalkylsilane is disclosed.
In the above Japanese Laid-Open Patent Publication No. 43247/1979, as can be seen also from the description in its specification that it is necessary to exercise care regarding acidity and dryness in preparation of its adhesive, when employing the above ultra-fine silica, namely, fumed silica, there appears retardation in the curing speed caused by its silicic acid behavior, which is a vital defect as an instantaneous adhesive. Also, according to Japanese Laid-Open Patent Publication No. 107993/1979, although a composition having good liquid properties can be obtained, the water resistance behavior of the cured product is not satisfactory.
According to the above two U.S. patent inventions, an adhesive improved to some extent in thixotropic, gelled performance and water resistance behavior of the cured product is obtained. However, when silica gel or fumed silica is treated with polydimethylsioxane or trialkoxyalkylsilane, the reaction does not proceed unless an acid is used as a catalyst, and also even if an acid is used as the catalyst, the hydrolysis reaction of the silicate moiety is liable to become nonuniform, whereby deviations are liable to occur in the treatment working of silica gel, thus entailing unsatisfactorily lack of reproducibility and stability of the physical properties of the product.